Employee and Agent Benefits (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 1,429.0	$ 1,417.7	$ 1,250.3
Qualified defined benefit pension plans
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,429.0	1,417.7
Qualified defined benefit pension plans | U.S. large cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	593.6	580.9
Qualified defined benefit pension plans | U.S. small/mid cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	139.0	143.5
Qualified defined benefit pension plans | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	216.5	241.7
Qualified defined benefit pension plans | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	347.8	331.5
Qualified defined benefit pension plans | Real estate investment trusts
Defined benefit plan, fair value of plan assets
Fair value of plan assets	37.4	35.4
Qualified defined benefit pension plans | Direct real estate investments
Defined benefit plan, fair value of plan assets
Fair value of plan assets	94.7	84.7
Qualified defined benefit pension plans | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1,429.0	1,333.0
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | U.S. large cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	593.6	580.9
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | U.S. small/mid cap equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	139.0	143.5
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	216.5	241.7
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | Fixed income security portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	347.8	331.5
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | Real estate investment trusts
Defined benefit plan, fair value of plan assets
Fair value of plan assets	37.4	35.4
Qualified defined benefit pension plans | Fair value hierarchy Level 2 | Direct real estate investments
Defined benefit plan, fair value of plan assets
Fair value of plan assets	94.7
Qualified defined benefit pension plans | Fair value hierarchy Level 3
Defined benefit plan, fair value of plan assets
Fair value of plan assets		84.7	54.0
Qualified defined benefit pension plans | Fair value hierarchy Level 3 | Direct real estate investments
Defined benefit plan, fair value of plan assets
Fair value of plan assets		84.7	54.0
Other postretirement benefits
Defined benefit plan, fair value of plan assets
Fair value of plan assets	466.6	471.7	421.5
Fair value of plan assets included in trust owned life insurance contract	51.6	54.1
Other postretirement benefits | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1.8	1.3
Other postretirement benefits | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	225.3	232.2
Other postretirement benefits | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	44.0	50.2
Other postretirement benefits | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	153.0	143.5
Other postretirement benefits | Principal Life general account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	42.5	44.5
Other postretirement benefits | Fair value hierarchy Level 1
Defined benefit plan, fair value of plan assets
Fair value of plan assets	372.5	373.1
Other postretirement benefits | Fair value hierarchy Level 1 | Cash and cash equivalents
Defined benefit plan, fair value of plan assets
Fair value of plan assets	1.8	1.3
Other postretirement benefits | Fair value hierarchy Level 1 | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	184.1	190.0
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	33.6	38.3
Other postretirement benefits | Fair value hierarchy Level 1 | Fixed income investment funds
Defined benefit plan, fair value of plan assets
Fair value of plan assets	153.0	143.5
Other postretirement benefits | Fair value hierarchy Level 2
Defined benefit plan, fair value of plan assets
Fair value of plan assets	51.6	54.1
Other postretirement benefits | Fair value hierarchy Level 2 | U.S. equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	41.2	42.2
Other postretirement benefits | Fair value hierarchy Level 2 | International equity portfolios
Defined benefit plan, fair value of plan assets
Fair value of plan assets	10.4	11.9
Other postretirement benefits | Fair value hierarchy Level 3
Defined benefit plan, fair value of plan assets
Fair value of plan assets	42.5	44.5	45.5
Other postretirement benefits | Fair value hierarchy Level 3 | Principal Life general account investment
Defined benefit plan, fair value of plan assets
Fair value of plan assets	$ 42.5	$ 44.5	$ 45.5